In early October, SpaceX founder Elon Musk debuted a new plan for colonising Mars with 1 million people.

The centrepiece of Musk’s roughly 42-minute talk was the “Big F-ing Rocket“, or BFR. Musk hopes to launch the first 35-story BFR toward the red planet by 2022.

But the billionaire tech mogul also teased a bonus use for the BFR: flying people anywhere in the world in less than one hour.

“If we’re building this thing to go to the moon and Mars, then why not go to other places on Earth as well?” Musk said during his presentation at the International Astronautical Congress.

The BFR design has two main sections: a rocket and a spaceship. The 191-foot-tall (58 metre) rocket would push the spaceship into orbit around Earth, then the 157-foot-long (48 metre) spaceship would fly about 100 people to Mars.

Everything would run on liquid methane and oxygen. The BFR would self-land and be fully reusable – a scheme that could slash the cost of access to space 1,000-fold.

The BFR’s spaceship could fly more than 4.6 miles (7.4 km) per second, according to SpaceX – over 12 times faster than the supersonic Concorde jets of yesteryear.

At that speed, a trip from Los Angeles to New York would take just 25 minutes, travelling from Bangkok to Dubai would take 27 minutes, passengers could get from London to New York in 29 minutes, and the flight from Delhi to San Francisco would last 40 minutes – “anywhere on Earth in under an hour,” according to a video Musk showed.

To understand what it may feel like to ride Musk’s giant spaceship, we asked former NASA astronaut Leroy Chiao.

What a BFR ride around the world would feel like

Chiao knows a thing or two about spaceflight, since he’s flown on three NASA space shuttles, a Russian Soyuz spacecraft to the International Space Station, and has lived nearly 230 days in space.

“What Elon Musk is describing would be a suborbital flight halfway around the world,” Chiao told Business Insider in an email.

Suborbital vehicles don’t orbit Earth. Instead, they make a fast and high arc through space and careen back toward the surface.

SpaceX/YouTube

NASA has a long history of launching them, and Virgin Galactic – Richard Branson’s company aerospace company – is now building and testing a suborbital vehicle called SpaceShipTwo.

“[L]aunch, insertion and entry would be similar to a capsule spacecraft [like the Soyuz], with the difference being in the final phase of landing,” Chiao said.

“During launch on a rocket with liquid engines … the liftoff is very smooth and one really can’t feel it.”

After the BFR (also called a first stage) runs out of fuel, the spaceship would separate from the rocket and fire its own engines.

Chiao said this moment would feel “a bit dynamic,” describing the experience in terms of g-force – the equivalent of gravity at Earth’s surface multiplied by a certain amount.

“Ignition of the next stage engine(s) causes a momentary bump in g-force,” he said. “As you get to the last part of ascent, you feel some g’s come on through your chest, but it is not uncomfortable.”

When the spaceship’s engines cut off, though, Chiao said you’d become “instantly weightless” as you temporarily coast through space.

“You feel like you are tumbling, as your balance system struggles to make sense of what is happening, and you are very dizzy,” he added. “You feel the fluid shift [in your body], kind of like laying heads-down on an incline, because there is no longer gravity pulling your body fluids down into your legs. All this can cause nausea.”

This feeling is familiar to anyone who’s drifted over a hill on a roller coaster, or flown on a parabolic “zero-gravity” flight – often referred to as a “vomit comet” ride, due to the intense nausea the experience can trigger.

“As you start to re-enter the atmosphere, you would feel the g’s come on smoothly and start to build,” Chiao said. As the spaceship noses up and down to shed speed, he added, at points you’d feel about 5G – a force that would make you feel roughly five times heavier than normal.

As the spaceship speeds toward the ground, its engines would fire to land itself on a floating barge.

“[Y]ou would both feel and hear [the engines],” Chiao said. “As the thrust builds, you would feel the g’s come on again and then at touchdown, you would feel a little bump.”

As exciting as such a trip might be – and the hours of flying it’d save – Chiao said it wouldn’t be for everyone.

“[T]his would not be for the faint of heart, and it is difficult to see how this would be inexpensive,” he added. “But the one thing I’ve learned from observing Elon, is not to count him out.”